Nigeria's President-elect Muhammadu Buhari vowed on Tuesday to make every effort to free more than 200 schoolgirls abducted by Boko Haram militants a year ago but admitted it was not clear whether they would ever be found.

The abduction of the girls from a secondary school in Chibok in the country's Northeast last April drew international attention to the humanitarian crisis caused by Boko Haram's attempt to establish a caliphate in religiously mixed Nigeria, Africa's biggest oil producer.

A march is expected in the capital, Abuja, on Tuesday to mark the one-year anniversary of the mass kidnapping.

Buhari, whose presidential election win two weeks ago was the first democratic defeat of an incumbent in Africa's biggest economy and most populous nation, said his administration would do everything it could to defeat the militant Islamist group.

"We do not know if the Chibok girls can be rescued. Their whereabouts remain unknown. As much as I wish to, I cannot promise that we can find them," he said in a statement.

"My government will do everything in its power to bring them home," added the former military ruler, who said his approach would differ from that taken by President Goodluck Jonathan.

Jonathan was criticized for a slow response to the Chibok girls crisis after he argued that a hasty rescue risked killing them.

"It was handled in a lousy manner," Lawan Abana, whose two nieces were among those kidnapped, said by telephone.

"We are confident that the girls are still alive," said Abana, his voice trembling and pausing after each word.

Amnesty International said in a report on Tuesday that Boko Haram has kidnapped at least 2,000 Nigerian women and girls since the start of 2014, many of whom were sexually abused or trained to fight.

The document, which includes scores of victim testimonies, accuses Boko Haram of rape, forced marriages and coercing them into armed attacks, sometimes on their own villages.

Female suicide bombers have been used by the insurgents in a spate of bombings in the last few months.

"I was among the girls trained to shoot. I was also trained how to use bombs and how to attack a village," a 19-year-old woman, who did not want to be identified, told Amnesty. She also described repeated rapes by gangs of up to six men.

With help from Nigeria's neighbors Niger, Cameroon and especially Chad, the rebels have been forced to retreat from an area the size of Belgium in the last few weeks.